Pulses of signals in the so-called “terahertz region”, which is also known as the “terahertz gap”, e.g., between 300 gigahertz and 10 terahertz, are useful for various applications, e.g., spectrum analyzer and imaging applications. Prior art terahertz pulse wave generators employed a Titanium-sapphire (Ti:Sapphire) laser which generates pulses of light, typically at a carrier wavelength of 780 nm corresponding to approximately 400 terahertz, each pulse having a period typically of between 50 and 300 femtoseconds, i.e., the spectral width of each pulse is approximately 3 terahertz. The pulses are then directed to an optical splitter, which generates two replicas of the pulses. The pulses of one of the replicas is supplied to a biased Auston switch, which responds to the input optical pulses to produce electromagnetic pulses with a spectral width of approximately one to two terahertz. The Auston switch includes an antenna, and possibly a lens, e.g., a silicon lens, to focus the terahertz pulse.
One use of such terahertz pulses is made by having the terahertz pulses being directed at a material under test, e.g., a pharmaceutical product, so that they are reflected back therefrom to a receiving, unbiased Auston switch. The unbiased Auston switch is supplied with a variably delayed version of the second replica of the optical pulses generated by the Titanium-sapphire laser. Typically, a mechanically tunable optical delay line implements the delay. The unbiased Auston switch generates an electrical output in response to a reflected terahertz signal that it receives when it is stimulated by a terahertz optical signal.
The delayed optical pulses of the second replica are used to control the time at which an output is produced by the Auston switch. In other words, the delayed optical pulses of the second replica “gates” the output of the Auston switch, an output being generated therefrom only when an optical pulse of the second replica is received. The delay is changed so that the part of the reflected terahertz pulse to be investigated arrives at the Austin switch as the same time as the gating pulse. Note that each reflected pulse should be the same provided that no changes are made to the first replica or the location of the material under test. Thus, to gain an overall impression of the entirety of a reflected terahertz pulse, the delay in the path of the second replica is changed for each of a sequence of terahertz pulses to scan over the entire width of one of the reflected pulses.
Disadvantageously, the use of a Titanium-sapphire laser is unduly expensive. Also, use of the mechanically tunable delay line makes the measurement slow and relatively expensive. Furthermore, because the delay line is mechanical, the device is less robust, and hence not as suitable to mobile applications, than might be desired.